1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet (copy sheet, transfer sheet, photosensitive sheet, electrostatic recording sheet, print sheet, OHP sheet, envelope, post card, sheet original or the like) placed on a sheet containing portion (sheet stacking platform, sheet stacking tray, sheet stacking deck, removable sheet supply cassette, manual sheet supply platform or the like) or a sheet manually supplied from the sheet containing portion one by one to a sheet treatment portion such as an image forming station, exposure station, treating station or the like in an image forming system such as a copying machine, facsimile and the like or a recording system (printer) acting as an information output equipment of a word processor, personal computer and the like.
2. Related Background Art
For conconvenience' sake, an example of a sheet feeding apparatus of a printer shown in FIG. 4 will be explained.
A sheet support plate (sheet guiding means) 6 acting as a sheet containing portion (sheet stacking means) is disposed so that a front end thereof is inclined downwardly. An urging plate (intermediate plate) 8 is disposed above an upper surface of the sheet support plate and is always floated from the upper surface of the plate 6 by a biasing force of a spring member 8a. Sheet separating pawls (sheet separating means) 7 are arranged at front corners of the sheet support plate for separating a single sheet from the other sheets. The sheets 5 (copy sheets or recording media) are stacked on the sheet support plate 6 so that leading ends of the sheets are regulated or locked by the separating pawls 7.
A sheet supply roller 9 acting as a sheet supply means serves to afford a feeding force to the sheets stacked on the plate 6 and comprises a shaft portion 9a and a roller portion 9b integrally formed with the shaft portion. An uppermost sheet on the sheet stack 5 stacked on the plate 6 is urged against the roller portion 6a of the sheet supply roller 9 by the urging plate 8 biased upwardly by means of the spring member 8a.
A sheet feed roller (sheet relay convey means) 16 is arranged ahead of the sheet support plate 6 in a sheet feeding direction and comprises a shaft portion 16b and a roller portion 16a integrally formed with the shaft portion.
A sheet guide plate 26 is disposed between the sheet support plate 6 and the sheet feed roller 16 so that a leading end thereof is inclined downwardly, which sheet guide plate serves to guide the sheet 5 below the sheet feed roller 16. The leading end portion of the sheet guide plate 26 is arcuated to conform with a lower half surface of the roller portion 16a of the sheet feed roller 16 and to extend to the left side of the sheet feed roller.
First and second pinch rollers 17A and 17B are urged against the lower portion of the sheet feed roller 16 by respective spring members (not shown) at two upstream and downstream points along the sheet feeding direction, respectively. These pinch rollers are contacted with the sheet feed roller 16 through openings 26a formed in the arcuated leading end portion of the sheet guide plate 26, respectively, and are driven by the rotational movement of the sheet feed roller 16.
A platen bar 15 is disposed tangentially to the sheet feed roller 16 in the vicinity of the latter at the left side thereof. A reciprocable carriage 11 can be reciprocally shifted in parallel with the platen bar 15 by means of a guide rail and a drive means (both not shown). A recording head 12 and an ink ribbon cassette 13 are mounted on the carriage 11, and the recording head 12 is opposed to the platen bar 15 with the interposition of an ink ribbon 14.
When the sheet supply roller 9 is rotated in a clockwise direction, the uppermost sheet on the sheet stack 5 stacked on the sheet support plate 6 is subjected to the sheet feeding force, with the result that the front corner portions of the uppermost sheet ride on the separating pawls 7 to be unlocked by the separating pawls, thus separating the uppermost sheet alone from the other sheets. The separated uppermost sheet is guided by the sheet guide plate 26 to reach a nip between the sheet feed roller 16 and the first pinch roller 17A.
The uppermost sheet 5 is fed by the sheet feed roller 16 and the first pinch roller 17A between the arcuated end portion of the guide plate 26 and the lower surface of the sheet feed roller 16, and then is fed by the sheet feed roller 16 and the second pinch roller 17B between the arcuated end portion of the guide plate 26 and the lower surface of the sheet feed roller 16, so that the leading end of the sheet enters into a space between the platen bar 15 and the ink ribbon 14.
When a predetermined amount of the sheet is entered into the space between the platen bar 15 and the ink ribbon 14, the rotational movement of the sheet feed roller 16 is changed to an intermittent rotational drive control wherein the sheet is fed by one printing line space, and the control of the reciprocal shifting movement of the carriage 11, head-down/head-up control of the recording head 12, the feed control of the ink ribbon 14 and the like are executed in co-relation with each other by means of a record control circuit (not shown), thus performing the recording operation with respect to the sheet 5 per one line.
In consideration of the cost-down of the apparatus, the actuation of the sheet supply roller 9 may be linked with the activation of the sheet feed roller 16 by means of a feed motor (not shown). In this case, a clutch is disposed between the sheet supply roller 9 and the feed motor to switch over the activation between the sheet supply roller and the sheet feed roller. It is a most simplified method that the switching of the clutch is effected by rotating the feed motor in a direction opposite to a normal direction in which the feed motor is rotated when the sheet is supplied.
In such a method, when the feed motor is rotated in the normal direction by a sheet supply start signal, a normal rotational force of the motor is transmitted to the sheet supply roller 9 through the clutch, so that the sheet supply roller 9 is rotated in a sheet feeding direction to separate and feed the uppermost sheet from the sheet stack 5. The sheet feed roller 16 is also rotated in the sheet feeding direction.
The leading end of the sheet 5 is sent to the nip between the sheet feed roller 16 and the first pinch roller 17A by the rotation of the sheet supply roller 9. When the sheet is sent by a predetermined length or distance after the leading of the sheet has just passed through the nip, the feed motor is switched to be rotated reversely.
The clutch connection between the feed motor and the sheet supply roller 9 is disengaged by the reverse rotation of the feed motor, thus stopping the sheet supply roller 9. The sheet feed roller 16 is rotated in a reverse direction Q opposite to the sheet feeding direction P, so that the sheet fed by the predetermined distance through the nip between the feed roller 16 and the first pinch roller 17A is fed back until the leading end of the sheet passes through the nip between the feed roller 16 and the first pinch roller 17A.
By feeding back the leading end of the sheet in this way, a bent loop (as shown by the solid line) is formed in a sheet portion between the stationary sheet supply roller 9 and the nip (between the feed roller 16 and the first pinch roller 17A) in opposition to the resiliency of the sheet.
By forming such bent loop in the sheet, the leading end of the sheet is urged against the nip between the feed roller 16 and the first pinch roller 17A due to the reaction force of the bent loop, with the result that any skew-feed of the sheet is corrected to register the leading edge of the sheet with a longitudinal direction of the feed roller 16.
Then, by rotating the feed motor in the normal direction again, the leading end of the sheet which was registered with the longitudinal direction of the feed roller 16 is re-entered into the nip between the sheet roller 16 rotating in the normal direction P and the first pinch roller 17A urged against the sheet feed roller, thus feeding the sheet 5 to the recording head 12 without skewing the sheet.
However, in such a sheet feeding apparatus, if a kind of sheets is changed or the resiliency of the sheet is increased due to the change in the temperature and/or humidity in the apparatus, when the sheet feed roller 16 is rotated in the reverse direction, the resiliency of the sheet portion between the reverse rotating feed roller 16 and the stationary sheet supply roller 9 may overcome the sheet feeding-back force generated by the reverse rotating feed roller 16.
In such a case, there arises a relative slipping movement between the leading end portion of the sheet pinched between the feed roller 16 and the first pinch roller 17A and the reverse rotating feed roller 16, thus preventing the sheet from being fed back. Consequently, the skew-feed of the sheet cannot be corrected (because the loop cannot be formed in the sheet portion) and the sheet is distorted or damaged by the relative slipping movement between the sheet and the reverse rotating feed roller 16.
Further, if the resiliency of the sheet is too strong or the sheet is skew-fed into the apparatus, there arose a problem that the bent loop formed in the sheet portion during the reverse rotation of the feed roller becomes non-uniform and/or the sheet is non-uniformly depressed between the feed roller 16 and the first pinch roller 17A, thus remaining the skew-fed condition of the sheet.